1. Field of the Invention
The present invention relates to a method of improving the audibility of sound from a loudspeaker located close to an ear. It relates particularly, though not exclusively, to an improved method of providing acoustic noise reduction in telephones and headphones, such that the listener can perceive the transmitted sounds more effectively in a noisy environment. It also relates to apparatus for use in the method.
2. Discussion of the Related Art
The principle of acoustic noise cancellation is well known. The unwanted, incoming noise signal is received by a transducer such as a microphone; the resultant signal is inverted and broadcast via a loudspeaker, or similar actuator, so as to combine with the original acoustic noise signal at a point set slightly apart from the pickup microphone. (The paper “The active control of sound”, S J Elliott and P A Nelson, Electronics and Communication Engineering Journal, August 1990, pp. 127–136, contains a useful reference listing.) It has been employed with varying success over many years for a variety of applications. For example, it has been speculated that large-scale noise cancellation might be used to quiet industrial office environments, but with little success. The problem is that whilst it is theoretically possible to cancel noise at a single point in space, the cancellation does not persist at other points in space because the magnitude and phase of the unwanted signals cannot be matched. Indeed, the cancellation signal itself contributes to the overall noise level away from the cancellation spot.
Known noise-reduction systems can be divided into two categories: (a) systems based on loudspeakers located far from the ear, and (b) systems based on loudspeakers located close to or adjacent the ear (including headphone-based systems, such as that disclosed in WO95/00946). The present invention relates to the latter category.
In the early 1990s, car makers were considering building noise-cancellation systems into the headrests of their cars. This is a sensible approach, because the cancellation spot is well-defined (i.e., at each respective ear of the driver), and directly accessed by a loudspeaker several inches away, such that the magnitude and phase of the signals arriving at the ear are well-defined. A similar system was contemplated for aircraft.
In general, low frequency noises are easier to cancel than high frequency ones, because the wavelengths are longer and hence the amplitude of the wave does not change as much per unit distance as it does for short wavelengths.
More recently, Sony have introduced noise-cancelling headphones under the WARP trademark in Japan. WARP stands for Wave Adaptive Reduction Principle. These are available in two forms: (a) conventional circumaural style, where the driver unit sits on a padded collar which lies around the edge of the pinna, and (b) in-ear style. More recently, Sony have also introduced a “pad-on-ear” type headphone (model MDR-NC5).
Other, similar products are made by, for example, Sennheiser, NCT, and Koss. Sennheiser manufacture several types of active, noise-cancelling headphones, such as their HDC 451, and NCT (of Stamford, USA) have a “Noisebuster” headphone system.
FIG. 1 shows a known means of acoustic noise-reduction, comprising a headphone driver unit (one of a pair), mounted in a headphone shell, together with an outer, noise-pickup microphone. The microphone registers the background noise on the outer side of the headphone shell assembly, and feeds an inverting amplifier. The output from this inverting amplifier is added to (i.e., summed with) the music signals at the headphone driver stage, such that the resultant noise-related signal on the inside of the headphone shell is equal and opposite to that of the background noise which permeates directly through and around the headphone shell.